In-situ short-circuit protection system and method for high-energy electrochemical cells

ABSTRACT

An in-situ thermal management system for an energy storage device. The energy storage device includes a plurality of energy storage cells each being coupled in parallel to common positive and negative connections. Each of the energy storage cells, in accordance with the cell&#39;s technology, dimensions, and thermal/electrical properties, is configured to have a ratio of energy content-to-contact surface area such that thermal energy produced by a short-circuit in a particular cell is conducted to a cell adjacent the particular cell so as to prevent the temperature of the particular cell from exceeding a breakdown temperature. In one embodiment, a fuse is coupled in series with each of a number of energy storage cells. The fuses are activated by a current spike capacitively produced by a cell upon occurrence of a short-circuit in the cell, thereby electrically isolating the short-circuited cell from the common positive and negative connections.

This application is a divisional of application Ser. No. 08/900,929,filed Jul. 25, 1997 now U.S. Pat. No. 6,099,986. The application isincorporated herein by reference.

GOVERNMENT LICENSE RIGHTS

The Government of the United States of America has rights in thisinvention pursuant to Cooperative Agreement No. DE-FC02-91CE50336awarded by the U.S. Department of Energy.

FIELD OF THE INVENTION

This invention relates generally to energy storage devices, and moreparticularly, to an apparatus and method for protecting energy storagecells upon occurrence of a short-circuit condition.

BACKGROUND OF THE INVENTION

The demand for new and improved electronic and electro-mechanicalsystems has placed increased pressure on the manufacturers of energystorage devices to develop battery technologies that provide for highenergy generation in a low-volume package. Conventional battery systems,such as those that utilize lead acid for example, are often unsuitablefor use in high-power, low-weight applications. Other known batterytechnologies may be considered too unstable or hazardous for use inconsumer product applications.

A number of advanced battery technologies have recently been developed,such as metal hydride (e.g., Ni—MH), lithium-ion, and lithium polymercell technology, which would appear to provide the requisite level ofenergy production and safety margins for many commercial and consumerapplications. Such advanced energy storage systems, however, typicallyproduce a significant amount of heat which, if not properly dissipated,can result in a thermal runaway condition and eventual destruction ofthe cells, as well as the system being powered by the cells.

The thermal characteristics of an advanced battery cell must thereforebe understood and appropriately considered when designing a batterysystem suitable for use in commercial and consumer devices and systems.A conventional approach of providing a heat transfer mechanism externalto such a cell, for example, may be inadequate to effectively dissipateheat from internal portions of the cell. Such conventional approachesmay also be too expensive or bulky in certain applications. The severityof consequences resulting from short-circuit and thermal run-awayconditions increases significantly when advanced high-energyelectrochemical cells are implicated.

There is a need in the advanced battery manufacturing industry for anenergy storage system that exhibits high-energy output, and one thatprovides for safe and reliable use in a wide range of applications.There exists a further need for a non-intrusive, inexpensive thermalmanagement approach that protects energy storage cells from thermalrun-away resulting from a short-circuit condition. The present inventionfulfills these and other needs.

SUMMARY OF THE INVENTION

The present invention is directed to an in-situ thermal managementsystem for an energy storage device. The energy storage device includesa plurality of energy storage cells each being coupled in parallel tocommon positive and negative connections. Each of the energy storagecells, in accordance with the cell's technology, dimensions, andthermal/electrical properties, is configured to have a ratio of energycontent-to-contact surface area such that thermal energy produced by ashort-circuit in a particular cell is conducted to adjacent andneighboring cells so as to prevent the temperature of the particularcell from exceeding a breakdown temperature. In one embodiment, a fuseis coupled in series with each of a number of energy storage cells. Thefuses are activated by a current spike capacitively produced by a cellupon occurrence of a short-circuit in the cell, thereby electricallyisolating the short-circuited cell from the common positive and negativeconnections.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B illustrate an embodiment of a solid-state, thin-filmelectrochemical cell having a prismatic configuration and including athermal conductor in accordance with an embodiment of the presentinvention;

FIG. 1C is a partial illustration of an energy storing module containinga stack of thin-film electrochemical cells and employing an in-situthermal management methodology in accordance with an embodiment of thepresent invention;

FIG. 2 is a graphical representation of a relationship between voltageand capacity for an electrochemical cell of the type illustrated in FIG.1;

FIG. 3 is an illustration of various film layers constituting athin-film electrochemical cell;

FIG. 4 illustrates various energy storage device configurations;

FIG. 5 is an illustration of a grouping of energy storage cellssubjected to a temperature increase due to a short-circuit condition inone of the cells;

FIG. 6 is a graphical representation of a relationship between maximumtemperature of a cell under short-circuited conditions and normalizedenergy content of a cell, the graph providing ratios of energycontent-to-contact surface area for adjacently disposed cells;

FIGS. 7-9 illustrate various cell configurations that exhibit productiveratios of energy content-to-contact surface area;

FIG. 10 shows an embodiment of a multiple-cell energy storage device inwhich one of the cells is subject to a short-circuit condition;

FIG. 11 illustrate a relationship between the maximum temperature in acell stack as a function of the number of adjacent short-circuited cellsat five difference state of charge (SOC) levels;

FIG. 12 illustrates a characteristic current waveform for anelectrochemical cell upon occurrence of a short-circuit in the cell;

FIG. 13 is an embodiment of an integrated short-circuit protectiondevice in accordance with an embodiment of the present invention;

FIG. 14 is an exploded view of an energy storing module containing anumber of interconnected thin-film electrochemical cells;

FIG. 15 is a cross-sectional illustration of an embodiment of a pressuregenerating apparatus for maintaining a stack of electrochemical cells ina state of compression;

FIG. 16 is an illustration of a band or strap including a tensionproducing clamp for use in a pressure generating apparatus formaintaining a stack of electrochemical cells in compression duringcharge and discharge cycling;

FIG. 17 is a perspective view of the tension producing clamp shown inFIG. 16; and

FIGS. 18-19 illustrate in a graphical form a relationship betweenmaximum cell temperature of an energy storing module and the energycontent and thickness of the cell, respectively.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In accordance with one embodiment of an energy storage system thatutilizes high-energy electrochemical cells, the system includessolid-state, thin-film cells of the type shown in FIG. 1. Such thin-filmelectrochemical cells are particularly well-suited for use in theconstruction of high-current, high-voltage energy storing modules andbatteries, such as those used to power electric vehicles for example.

In FIG. 1A, there is shown an embodiment of a prismatic electrochemicalcell 50 which includes an anode contact 56 and a cathode contact 55formed respectively along opposing edges of the electrochemical cell 50.A thermal conductor 52 is spot welded or otherwise attached to each ofthe anode and cathode contacts 56, 55, respectively. The thermalconductor 52 is typically disposed along the length of the anode contact56 and the cathode contact 55, and typically includes an electricalconnection lead 54 for conducting current into and out of theelectrochemical cell 50, the current being collected and conductedpreferentially along the anode and cathode contacts 56, 55.

The embodiment of a thermal conductor 63 shown in FIG. 1B includes acopper tab 53 that extends along the length of a sprayed metal anode orcathode contact 61. The copper tab 53 includes a resilient member 59through which heat is transferred between the cell 50 and an adjacentlydisposed heat sink, such as a wall of a metallic housing. The copper tab53 is spot welded to the sprayed metal contact 61 at a number of weldlocations 51. A flexible electrical lead 57 is ultrasonically welded tothe end 63 of the copper tab 53. Current is conducted primarily alongthe sprayed metal contact 61 of the cell 50 and communicated to externalconnections via the flexible electrical leads 57.

As is shown in FIG. 1C, the thermal conductor 63 provides a thermal fluxpath for transferring thermal energy between the electrochemical cellsand a thermally conductive, electrically resistive material or element.It is to be understood that a thermally conductive, electricallyresistive material, element or structure as described herein refers to asurface coating/treatment or separate material that permits a sufficientamount of heat to be conducted therethrough, yet is electricallyresistive to the flow of current relative to a current path provided forconducting current into and out of an electrochemical cell. An anodizedcoating, for example, may have a thickness that permits a sufficientamount of thermal energy to be conducted therethrough, yet issufficiently resistive to electrical current relative to the anode andcathode contacts of the cell or the thermal conductor. By way of furtherexample, a thermally conductive polymer element may be employed, withthe density of thermally conductive particles impregnated therein beingselected to provide a desired balance between thermal and electricalconductivity characteristics.

As is further shown in the multiple cell embodiment of FIG. 1C, thethermal conductors 63 also provide a thermal flux path for transferringheat between neighboring cells. If a short develops in a cell 73 withina stack of cells, for example, the excess heat, Q_(gen), generated bythe short-circuited cell 73 is conducted through the thermallyconductive, electrically resistive material provided on the housingsurface 77, and to adjacent cells 72 and non-adjacent neighboring cells71 via the thermal conductors 63. The excess heat, Q_(gen), is alsoconducted to adjacent cells 72 in physical contact with theshort-circuited cell 73. A thermally conductive plate 75 serves as aheat sink for a cell 74 situated at the end of the cell stack.

Further, the thermal conductor 63 is configured so as to exhibit aspring-like character which provides for substantially continuouscontact between a cell 73 and a structure, such as a metallic planarsurface 77, disposed adjacent the cell 73 in response to relativemovement between the cell 73 and the adjacent structure 77. A separatespring element, 69, such as a tubular elastomeric element, may beretained within the thermal conductor 63 to enhance the springproperties of the thermal conductor 63. The thermal conductor 63 may befashioned from copper and have a substantially C-shaped, doubleC-shaped, z-shaped, O-shaped, S-shaped, V-shaped, or finger-shapedcross-section. Other useful thermal conductors are disclosed inco-pending patent application Ser. No. 08/900,428, now U.S. Pat. No.6,117,584 entitled “Thermal Conductor for High-Energy ElectrochemicalCells” (Hoffman et al.), the contents of which are incorporated hereinby reference.

In the embodiment shown in FIG. 1A, the electrochemical cell 50 isfabricated to have a length L of approximately 135 mm, a height H ofapproximately 149 mm, and a width W_(ec) of approximately 5.4 mm orapproximately 5.86 mm when including a foam core element. The widthW_(c) of the cathode contact 55 and the anode contact 56 isapproximately 3.72 mm, respectively. Such a cell 50 typically exhibits anominal energy rating of approximately 36.5 Wh, a peak power rating of87.0 W at 80 percent depth of discharge (DOD), and a cell capacity of14.4 Ah at full charge. FIG. 2 illustrates in graphical form arelationship between voltage and capacity for an electrochemical cellhaving a construction substantially similar to that shown in FIG. 1A. Itcan be seen that an individual electrochemical cell has a nominaloperating voltage ranging between approximately 2.0 V and 3.1 V.

The electrochemical cells shown in FIGS. 1A-1C may have a constructionsimilar to that illustrated in FIG. 3. In this embodiment, anelectrochemical cell 60 is shown as having a flat wound prismaticconfiguration which incorporates a solid polymer electrolyte 66constituting an ion transporting membrane, a lithium metal anode 64, avanadium oxide cathode 68, and a cathode current collector 70. Thesefilm elements are fabricated to form a thin-film laminated prismaticstructure, which may also include an insulation film, such aspolypropylene film.

The cell shown in FIG. 3 includes a central cathode current collector 70which is disposed between each of the two cathode films 68 to form abi-face cell configuration. A mono-face cell configuration mayalternatively be employed in which a single cathode collector 70 isassociated with a single anode/electrolyte/cathode element combination.In this configuration, an insulating film is typically disposed betweenindividual anode/electrolyte/cathode/collector element combinations.

A known sputtering metallization process is employed to form currentcollecting contacts along the edges 65, 79 of the anode and cathodecurrent collecting films 64, 70, respectively. It is noted that themetal-sprayed contacts provide for superior current collection along thelength of the anode and cathode film edges 65, 79, and demonstrate goodelectrical contact and heat transfer characteristics. Theelectrochemical cells illustrated in FIGS. 1A-1C and 3 may be fabricatedin accordance with the methodologies disclosed in U.S. Pat. Nos.5,423,110, 5,415,954, and 4,897,917.

In Table 1 below, various thermal properties are provided for anelectrochemical cell having a construction similar to that illustratedin FIG. 1 and maintained at a temperature of approximately 60° C.

TABLE 1 Thermal Conductivity (W/m ° C.) Direction Direction Specific ofthe film of the Density Heat Section thickness connectors (kg/m³) (J/kg° C.) Active Section 0.4042 48.10 1356 1411 Anode Side, 0.0466 28.90 252 2714 Inactive Zone Cathode Side, 0.0388 18.45  441 1470 InactiveSide Complete Cell 1218 1435 Other Components Density × ThermalConductivity specific heat Component (W/m ° C.) (kJ/m³ ° C.) Cell's core(foam) 0.071  401.3 Metallization 366.7 3254.6 Spring-type 134.5 3254.6conductor Vessel wall - 178.8 2566.9 anodized

A number of electrochemical cells may be selectively interconnected in aparallel and/or series relationship to achieve a desired voltage andcurrent rating. For example, and with reference to FIG. 4, a number ofindividual electrochemical cells 80 may be grouped together andconnected in parallel to common positive and negative power buses orlines to form a cell pack 82. A number of the electrochemical cell packs82 may then be connected in series to form a module 84. Further, anumber of individual modules 84 may be connected in series to constitutea battery 86.

The embodiment shown in FIG. 4 depicts an arrangement of electrochemicalcells 80 in accordance with a modular packaging approach which providesan efficient means of achieving desired power requirements for a broadrange of high-power applications. In this illustrative embodiment, eightelectrochemical cells 80 are grouped together and connected in parallelto form a cell pack 82. A module 84 is constituted by grouping six cellpacks 82 together and connecting the packs 82 in series. A battery 86 isshown as constituting 24 modules 84 connected in series.

Given these arrangements, and assuming that each of the electrochemicalcells 80 has dimensions and characteristics equivalent to those of thecell depicted in FIG. 1, each individual cell 80 provides for a totalenergy output of approximately 36.5 Wh. Each cell pack 82 provides for atotal energy output of approximately 292 Wh, while each module 84provides for a total energy output of 1.75 kWh. The battery 86,constituted by an array of four axially and six longitudinally orientedmodules 84 connected in series, provides for a total energy output ofapproximately 42 kWh. It is understood that the arrangement ofelectrochemical cells 80 and interconnection of cells 80 forming a cellpack 82, module 84, and battery 86, may vary from the arrangementsdepicted in FIG. 4.

In FIG. 5, there is shown a number of electrochemical cells arranged ina stack configuration. A particular cell 112 is depicted as havingsustained a short-circuit. The cell 112 generates heat as a consequenceof the high rate of energy discharge resulting from the short-circuit.In accordance with this one-dimensional (x-axis) heat conduction model,the thermal energy generated by the short-circuit in the cell 112 ispartially conducted through the cell 112 and to the-outer surfaces 115,117 of the cell 112. The close proximity of an adjacent cell 110 to theshort-circuited cell 112 permits the thermal energy conducted to theouter surfaces 115, 117 of the cell 112 to dissipate into the adjacentcell 110.

In a similar manner, an adjacent cell 114, having an outer surface 113in thermal contact with an outer surface 117 of the cell 112, conductsheat produced by the cell 112 through the thermal contact interface 113,117. In this illustrative example, the adjacent cells 110, 114 includeouter surfaces 111, 113 which are in intimate thermal contact with theouter surfaces 115, 117 of the cell 112. It is understood that an insertelement, such as a foam or metallic flat spring element, or thermallyconductive material, may be situated between adjacent cells. Althoughnot depicted in FIG. 5, it is understood that the heat generated by theshort-circuited cell 112 is also conducted in the y and z directionsand, in particular, to adjacent and neighboring cells via the thermalconductors and thermally conductive, electrically resistive material asis depicted in FIG. 1C.

It is believed that immediately following a short-circuit event in thecell 112, approximately 50% of the generated heat dissipates in thex-direction to adjacent cells 110, 114, while the remaining 50% isdissipated via the thermal conductors and thermally conductive,electrically resistive material. As time progresses, a disproportionateamount of the excess heat is dissipated via the thermal conductor route.It is noted that the end cells of the cell stack require the presence ofan adjacently situated heat sink, such as the metal plate 75 shown inFIG. 1C, which is in intimate contact with end cell 74.

Those skilled in the art will appreciate that the energy increase withinthe short-circuited cell 112, and the rate at which the energy generatedfrom the short-circuit event is dissipated into adjacent cells 110, 114,can be characterized through use of Fourier's Law of Heat Conduction. Indescribing a process by which heat generated from the short-circuitedcell 112 is conducted to adjacent cells 110, 114, a brief discussion ofa generalized one-dimensional heat conduction analysis may be useful. Itis understood that the following description is provided for purposes ofillustration only, and ignores three-dimensional transient heat transferconsiderations.

In the energy storage system illustrated in FIG. 5, the rate at whichheat flows axially through the short-circuited cell 112 is denoted asQ_(gen), which represents the heat generated per unit time in the cell112 of thickness dx. The heat conducted into the volume element 118 at alocation x=x₀ is given by the parameter Q_(x). The heat conducted out ofthe volume element 118 at a location x=x+dx is given by the parameterQ_(x+dx). In this simplistic description, the quantity Q_(gen)represents the heat energy generated throughout the volume element 118which is dependent on the rate of heat generation per unit volume perunit time, represented by the parameter {dot over (q)}, and the volumeof the element 118. The resulting energy balance equation is given by:

Q _(x) +Q _(gen) =Q _(x+dx)  1

and;

Q _(gen) ={dot over (q)}Adx  2

where, Qx, Q_(x+dx), and Q_(gen) represent heat flow rates measured inwatts (W), {dot over (q)} represents the rate of heat generation perunit volume per unit time measured in watts/m³, dx represents thethickness of the volume element 118, and A represents thecross-sectional area of the volume element 118.

Those skilled in the art will appreciate that a temperature increasewithin the energy storage system shown in FIG. 5 due to a short-circuitevent can be appropriately managed by understanding the thermalcharacteristics and energy producing capability of the cells. An in-situthermal management system in accordance with the principles of thepresent invention may be employed to dissipate excess thermal energyresulting from a short-circuit event without necessity of an externalactive thermal management scheme, such as a forced cooling or forcedconvection apparatus. The in-situ thermal management methodologydescribed herein may be implemented by understanding the heat capacityand heat dissipation characteristics of the particular cells used in anenergy storage system, and appropriately limiting the energy content ofthe cells.

An important consideration that impacts the design of a multiple-cellenergy storage system concerns the temperature at which the materials ofa particular cell technology break down or degrade such that overallcell performance is significantly reduced. By way of example, a cellhaving a construction of the type shown in FIGS. 1A-1C and 3 has abreakdown temperature of approximately 180° C., which represents themelting point of lithium. Employment of an in-situ thermal managementscheme implemented in accordance with the principles of the presentinvention prevents the temperature of a cell from reaching a breakdowntemperature, or a safety temperature lower than the breakdowntemperature, even under short-circuit conditions.

The heat dissipation characteristics of a particular cell are dependenton a number of factors, including the cell's technology, dimensions, andthermal/electrical properties. Taking into consideration these knownfactors, the heat dissipation characteristics of a cell may be alteredand optimized.

Since heat dissipation in the cell 112 is a function of thermal contactsurface area with respect to contact surfaces of adjacent cells 110,114, the maximum energy content per unit contact surface area requiredto maintain the cell temperature below a breakdown or safety temperaturemay be determined. By way of example, and with reference to FIG. 6,there is shown in graphical form a relationship between the maximumtemperature of a cell having a construction as shown in FIGS. 1A-1C and3 under short-circuit conditions and a ratio of normalized energycontent-to-contact surface area for the cell. It is to be understoodthat the graph of FIG. 6 characterizes a cell having a particularchemistry and having particular geometric and thermal/electricalproperties.

Using the graph shown in FIG. 6, the energy content of a cell and thephysical dimensions of the cell may be selected so that the ratio ofenergy content-to-cell surface area is kept within a range such that themaximum cell temperature remains below a breakdown or safetytemperature, even under short-circuit conditions. An energycontent-to-contact surface area ratio of less than approximately 0.0050Wh/cm² for a thin-film lithium polymer cell will ensure that aworst-case temperature resulting from a short-circuit in the cell doesnot exceed the melting point of the lithium elements within the cell(i.e., 180° C.).

If it desired to design the cell to ensure that a maximumshort-circuited cell temperature does not exceed a safety temperature,such as 130° C., the energy content and contact surface area of the cellmay be appropriately selected using the graph of FIG. 6. It isunderstood that graphs similar to that shown in FIG. 6 whichcharacterize maximum cell temperature under short-circuit conditionsrelative to the ratio of energy content-to-contact surface area may bedeveloped for energy storage cells constructed using technologies otherthan those described herein. It is noted that FIG. 18, for example,depicts a relationship between energy content and maximum celltemperature for a cell having a similar construction as that shown inFIGS. 1A-1C and 3 but a different cathode oxide.

The depictions of energy storage cells shown in FIGS. 7-9 are providedto illustrate that an in-situ thermal management design approach may beemployed for energy storage cells having varying configurations. Forexample, the length (L), height (H), width (w), or radius (r) may bevaried as needed for a given application, with the constraint that theratio of energy content-to-contact surface area remain in a range thatprevents the worst-case cell temperature from exceeding the cellbreakdown temperature.

In order to facilitate the proper design and manufacture of thermallystable energy storing modules and devices which contain a number ofclosely situated electrochemical cells of a given technology, it isuseful to express the maximum temperature achievable by the cells underworst-case conditions (i.e., a short-circuit) as a function of severalvariables, including the ratio of energy content of the cell to cellvolume, conductivity of the cells, thermal conductance, and cellthickness. The following equations characterize the maximum temperature,(T_(max)), of a short-circuited cell of a given technology when the cellis packaged in an energy storing module such as that depicted in FIGS.4, 10, and 14. It is noted that the equations below were developed byuse of numerical simulations of a multiple-cell module at an initialoperating temperature of 600° C. It is further, noted that theseequations were developed based on a cell technology implicated in FIG.18. Using the following equations, it is possible to calculate theconductance of a thermal conductor required to safely dissipate excessheat generated by a short-circuited cell.

Equation [3] below mathematically characterizes the maximum celltemperature of a thin-filmed electrochemical cell, which does notinclude a foam core element, as a function of various operativeparameters. The dimensions of the cell characterized in Equation [3] aregiven as 0.135 m×0.149 m×0.054 m. The maximum cell temperature for thecell is given by:

T _(max)=1/1.1·1/1.2·0.037738·(1/(ρ_(cell) ·Cp_(cell)))^(0.3856)·(Q/kcell)·(δ)^(0.6146)·(K/L)^(0.077)  3

where, T_(max) represents the maximum temperature reached by ashort-circuited cell in a module (° C.), ρ_(cell) represents the densityof the cell (kg/m³), Cp_(cell) represents the heat capacity of the cell(J/kgK), Q represents the energy content of one cell per unit volume(Wh/m³), kcell represents the conductivity of the cell in thecell-to-cell axial direction (W/mK), δ represents cell thickness in thecell-to-cell axial direction (mm), and K/L represents the conductance ofthe thermal conductor (W/m²K).

Using Equation [3] above, a relationship between maximum temperature ofa short-circuited cell as a function of the cell's energy content for agiven cell chemistry and configuration may be developed. A relationshipbetween maximum cell temperature as a function of cell thickness mayalso be developed. By way of example, and with reference to FIGS. 18-19,there is depicted a relationship between maximum cell temperature as afunction of energy content and cell thickness, respectively. The datareflected in FIGS. 18-19 was developed with the following variables heldconstant: kcell=0.4 W/mK, K/L=400 W/m²K, ρ_(cell)·Cp_(cell)=1218·1435J/m³K.

It can be seen from FIG. 18 that a thin-film electrochemical cell of thetype characterized above should have an energy content which is limitedto less than approximately 38 Wh to ensure that the maximum temperatureof the cell will not exceed a breakdown temperature, such as the meltingpoint of lithium (i.e., 180° C.). It is interesting to note thelinearity of the maximum cell temperature-to-energy content relationshipdepicted in FIGS. 18 and 6, given the difference in cell technology. Itcan be seen from FIG. 19 that the thickness of the cell should notexceed approximately 8.5 mm in order to ensure that the maximumtemperature of the cell does not exceed the 180° C. breakdowntemperature.

Equation [4] below characterizes maximum cell temperature for an energystoring module of the same cell technology as that implicated inEquation [3] in which some of the cells include a foam core elementcompressed to approximately 2 mm. More specifically, Equation [4]characterizes maximum cell temperature for a module design in whichcompressed foam core elements are provided in every two electrochemicalcells. In this case, maximum cell temperature for such a moduleconfiguration is given by:

T _(max)0.037738·(1/(ρ_(cell) ·Cp_(cell)))^(0.3856)·(Q/kcell)·(δ)^(0.6146)·(K/L)^(−0.077)  4

It is interesting to note that Equations [3] and [4] differ only byconstants (i.e., the constants 1/1.1 and 1/1.2 in Equation [3]).

Equation [5] characterizes the maximum cell temperature for a modulehaving cells of the same technology implicated in Equations [3]-[4],wherein the cells incorporate a foam core element that is thinner thanthe element associated with Equation [4] above. More specifically,Equation [5] below assumes that a foam core element having a thicknessof approximately {fraction (1/32)} inches is provided in every two cellsof the cell stack. The foam core element is fabricated from Poron S2000.The maximum cell temperature for a module having this configuration isgiven by:

T _(max)=1/1.1·0.037738·(1/(ρ_(cell) Cp_(cell)))^(0.3856)·(Q/kcell)·(δ)^(0.6146)·(K/L)^(−0.077)  5

It is noted that the term δ_(cell)·Cp_(cell) allows Equations [3]-[5] tobe used to quantify the effect of heat capacity of the components withinthe cell on the maximum cell temperature, T_(max), reached during ashort-circuit event. These equations, therefore, may be used tocharacterize maximum cell temperatures under similar situations forenergy storing cells of differing technologies.

These equations may also be employed to characterize the effects ofmodifications and improvements in cell design and construction. It isnoted that the numerical simulations used to develop Equations [3]-[5]were directed to the investigation of electrochemical cells having anenergy content that varied from approximately 30 to 40 Wh, a cellthickness, δ, that varies from approximately 5.4 and 7.8 mm, and cellsthat utilize a thermal conductor having a conductance value, K/L, thatvaries between approximately 200 and 600 W/m²K.

The in-situ thermal management approach described above with referenceto FIGS. 1C and 5 is generally applicable for managing short-circuittemperature increases occurring in a single cell of a grouping of cells.In applications in which a significant number of parallel connectedcells are configured in a stack or bundle, an enhanced in-situshort-circuit protection scheme may be implemented to prevent thermalrunaway within the cell stack, and to isolate a particular cell from theparallel connection upon occurrence of a short-circuit in the cell.

In the embodiment of an energy storage system illustrate in FIG. 10, theenergy storage device 120 includes eight energy storage cellsrespectively connected in parallel to common positive and negativeterminals 124, 125. The cell EC1 is shown as a short-circuit. Given thisarrangement, and with reference to FIG. 11, it can be seen that only oneshort-circuited cell within a stack of eight cells can be managed usingthe above-described in-situ thermal management methodology withoutexceeding the breakdown temperature of the cell material. An in-situshort-circuit protection device may be incorporated into an energystorage system to prevent multiple short-circuit events from occurring.

In accordance with one embodiment of the present invention, and as shownin FIG. 10, a fuse 123 is connected in series with a respective cell 122within the multiple-cell energy storage device 120. In the event that ashort-circuit occurs in any of the parallel connected cells 122, thefuse 123 of the defective cell 122 blows so as to electrically isolatethe short-circuited cell 122 from the parallel connection. The heatgenerated during development of the short-circuit in the cell 122 andafter blowing of the fuse 123 is conducted to cells adjacent thedefective cell 122 in a manner previously described. As such, themaximum temperature attainable by a cell under worst-case conditions iswell below the breakdown temperature of the cell. More particularly, thedata of FIG. 11 confirms that the temperature of a short-circuited cellwithin the cell stack never exceeds a safety temperature of 130° C. whenan in-situ short-circuit protection device is employed.

Referring now to FIG. 12, there is illustrated a graph whichcharacterizes the effect on cell current upon the occurrence of ashort-circuit in a thin-film electrochemical cell. A thin-film cell ofthe type shown in FIGS. 1A-1C and 3, as well as other types ofhigh-energy cells, exhibit a significant short-term increase in cellcurrent due to the capacitive characteristics of the cell. For example,the current in the cell characterized in FIG. 12 spikes at a value inexcess of 500 A in less than approximately 100 milliseconds. Followingthe current spike, the current in the cell rapidly decays toapproximately 150 A after 1 second, and gradually decays thereafter. At5 seconds following the short-circuit event, the cell current reaches avalue of approximately 60 A.

The characteristic current spike that occurs immediately after ashort-circuit event in a high-energy cell is advantageously exploited byan in-situ short-circuit protection device implemented in accordancewith the principles of the present invention. In the embodiment shown inFIG. 10, for example, each of the fuses 123 connected in series with acorresponding energy storage cell 122 are designed to activate inresponse to a current spike generated from a short-circuit in the cell122. A fuse 123 typically has a current rating that prevents the fusefrom activating during normal operation, yet permits the fuse toactivate in response to a short-circuit condition. Exploiting thecurrent spike as a triggering mechanism for the fuse 123 provides for alarge current gap between the maximum operating current level of thecell 122 and the minimum activation current level of the fuse 123.

In accordance with one embodiment, the parallel connected cells of anenergy storage device have a structure and behavior similar to thosepreviously described with reference to FIGS. 1A-1C and 3. In such aconfiguration, the fuses connected in series with the cells have acurrent rating of approximately 50 A. By utilizing the capacitive effectof the cell to trigger the 50 A fuse, unintentional activation of thefuse is avoided, providing for both safe and reliable short-circuitprotection of the energy storage device.

In some applications, protection against accidental shorting of anenergy storage device or cell, such as through a foreign conductiveimplement or material, may be of primary concern. It may be desirable,therefore, to employ a fuse that is activated more slowly than the fastacting fuse described above. For example, a fuse that activates afterseveral hundred milliseconds or several seconds after occurrence of ashort-circuit in the cell may be employed. Although excess heat isgenerated between the time the short occurs and the time the fuse blows,the in-situ thermal management methodology described previously providesfor the safe dissipation of such excess heat.

In FIG. 13, there is illustrated an embodiment of a short-circuitprotection device fabricated in an integrated package. The integrateddevice 130 includes an enclosure 132 within which eight fuses (notshown) are mounted. A first contact of each fuse is connected in serieswith a corresponding one of eight terminals 134, and a second contact ofthe each fuse is connected to a common bus 140. Each of the terminals134 includes a lead 136 and a contact 138. When the short-circuitprotection device 130 is connected to an array of cells, each of thecontacts 138 engages a corresponding contact of one of eight cells inthe array. The common bus 140 is typically coupled to one or more commonbusses of other short-circuit protection devices 130 connected tocorresponding cell arrays to form a series connected energy storagedevice, such as a module.

In one embodiment, the enclosure 132 has a height, H_(E), of 16.00 mm, awidth, W_(E), of 7.49 mm, and a length, L_(E), of 50.80 mm. The leadportion 136 of the terminal 134 has a height, H_(L), of 12.70 mm, awidth, W_(L), of 1.27 mm, and a length, L_(L), of 5.00 mm. The contactportion 138 of the terminal 134 has a height, H_(C), and a width, W_(C),of 1.27 mm, and a length, L_(C), of 13.03 mm. The common bus 140 has aheight, H_(CB), of 6.35 mm, a width, W_(CB), of 1.27 mm, and a length,L_(CB), of 49.02 mm.

In FIG. 14, there is shown an exploded view of an embodiment of anenergy storing module 142 which houses a number of electrochemical cells144, interconnection hardware, and control hardware and software. Inaccordance with one embodiment, the module 142 includes a stack of 48electrochemical cells 144 which are interconnected through use of ainterconnect board 147. Short-circuit protection circuitry, such as anintegrated short-circuit protection pack 148, is typically provided onthe interconnect board 147. Each of the six integrated short-circuitprotection packs 148 disposed on the interconnect board 147 electricallycouple to a corresponding one of six cell packs 143 upon mounting theinterconnect board 147 in place above the stack of cells 144.

The volume of an electrochemical cell of the type described previouslywith regard to FIG. 1 varies during charge and discharge cycling due tothe migration of lithium ions into and out of the lattice structure ofthe cathode material. This migration creates a corresponding increaseand decrease in total cell volume on the order of approximately five tosix percent during charging and discharging, respectively. In order toaccommodate variations in cell volume resulting from charge anddischarge cycling of a grouping of cells, a pressure producing apparatusis employed to maintain the cells in a continuous state of compressionto ensure continuous intimate contact between cell of the cell stack. Itis considered desirable that the compressive forces, whether producedinternally or externally of the cell, be distributed fairly uniformlyover the surface of application.

The stack of electrochemical cells 144 shown in FIG. 14 are bandedtogether by use of two bands 146 and two opposing thrust plates 145. The48 electrochemical cells 144 are subjected to continuous compressiveforces generated by use of the bands 146/thrust plates 145 and a foam orspring-type element disposed in each of the cells 144 and/or between allor selected ones of the cells 144. It is noted that the foam orspring-type core element provided in the center of each of the cells 144serves to distribute pressure evenly between the cells 144, which is ofparticular importance as cell volumes change during charge and dischargecycling.

In the embodiment illustrated in FIG. 15, a metal strap 194 includes awave-like spring 198 which generates tension forces that cause thethrust plates 194, in turn, to exert compressive forces on the cellstack 192. It is understood that the tension spring apparatusillustrated in FIG. 15 may be implemented using a number of coil springsor using elastomeric material, and that a combination of metallic andelastomeric spring materials may also be advantageously employed.Further, it will be appreciated that foam or other spring elements maybe incorporated within the cell stack and/or within individual cells incombination with a tension spring apparatus external to cell stack.

FIG. 16 illustrates an embodiment of a strap apparatus 180 which isparticularly useful in constraining a number of electrochemical cellsconfigured as a stack or bundle. In contrast to a strap apparatus whichis substantially non-extendible in its length, the strap apparatus shownin FIG. 16 incorporates a unique clamp 182 which significantly enhancesthe efficacy of a cell stack pressure system. The strap apparatusincludes two bands 180 each having C-shaped ends 181. A clamp 182 isattached to a band 180 by coupling the C-shaped ends 181 of the band 180with corresponding C-shaped ends 184 of the clamp 182. It is assumedthat the bands 180 are disposed around the stack of cells in a manner asshown in FIG. 15. The clamp 182 includes a hinge 186 integral to theclamp 182 which is collapsible onto a contact surface 188 of the clamp182 when subjected to sufficient force.

When the hinge 186 is collapsed onto the contact surface 188, theC-shaped ends 184 of the clamp 182 are pulled towards each other which,in turn, produces a tension force in the C-shaped ends of the bands 180.The magnitude of the tension force induced in the bands 180 by actuationof the clamps 182 is moderated by a sign wave-shaped spring 189 integralto the clamps 182. The sign wave-shaped spring 189 may be configured, interms of shape, thickness, and material, to provide for a desired amountof expansion and retraction of the strap apparatus duringcharge/discharge cycling of the cells. Other useful pressure generatingmechanisms are disclose in co-pending patent application Ser. No.08/900,429 entitled “Pressure System and Method for RechargeableThin-Film Electrochemical Cells” (Hoffman et al.), the contents of whichare incorporated herein by reference.

It will, of course, be understood that modifications and additions canbe made to the various embodiments discussed hereinabove withoutdeparting from the scope or spirit of the present invention. By way ofexample, a short-circuit protection device may include thermallyactivated fuses, such as Model NTE8090 manufactured by NTE Electronics,rather those described herein. Thermally activated fuses typicallyactivate at a prescribed temperature, such as a temperature below abreakdown temperature. Also, a thermally activated fuse may be connectedin series with a current activated fuse which provides for increasedactivation reliability. Further, the principles of the present inventionmay be employed for use with battery technologies other than thoseexploiting lithium polymer electrolytes, such as those employing nickelmetal hydride (Ni—MH), lithium-ion, (Li—Ion), and other high energybattery technologies. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments discussed above, butshould be defined only by the claims set forth below and equivalentsthereof.

What we claim is:
 1. A method of providing short-circuit protection fora plurality of parallel connected energy storing cells, comprising:maintaining the energy storing cells in thermal contact with oneanother; generating current using the energy storing cells; electricallyisolating a short-circuited cell of the plurality of cells in responseto a current spike produced by the short-circuited cell; and conductingheat generated by the short-circuited cell to other ones of theplurality of energy storing cells so that a temperature of theshort-circuited cell remains below a breakdown temperature.
 2. Themethod of claim 1, wherein electrically isolating the short-circuitedcell comprises blowing a fuse connected in series with theshort-circuited cell.
 3. The method of claim 2, wherein the fuse isblown in less than 100 milliseconds.
 4. The method of claim 1, whereinthe current spike produced by the short-circuited cell has an amperagein the range of approximately 300 A to 600 A.
 5. The method of claim 1,wherein maintaining the energy storing cells in thermal contact with oneanother comprises maintaining the energy storing cells in a state ofcompression.
 6. The method of claim 1, wherein the breakdown temperaturerepresents a melting temperature of the energy storing cells.
 7. Themethod of claim 1, wherein each of the energy storing cells compriseslithium, and the breakdown temperature represents a melting temperatureof lithium.
 8. The method of claim 1, wherein each of the energy storingcells has a prismatic configuration.
 9. The method of claim 1, whereineach of the energy storing cells has a ratio of energycontent-to-contact surface area of less than about 0.006 Wh/cm².
 10. Themethod of claim 1, wherein each of the energy storing cells has athickness that varies between about 3 mm to 10 mm and a ratio of energycontent-to-contact surface area of less than about 0.006 Wh/cm². 11.Themethod of claim 1, wherein each of the energy storing cells has asurface area ranging between about 100 cm² and 400 cm².
 12. The methodof claim 1, wherein each of the energy storing cells has a surface arearanging between about 100 cm² and 400 cm² and an energy content rangingbetween about 10 Wh and 40 Wh.